The knee orthosis of the present invention relates to orthotic devices generally, and specifically to a knee orthosis designed to allow normal range of knee movement while simultaneously providing protection against injury that may be caused by hyperextension or other harmful stresses to the knee joint.
The great increase in the popularity of sports, both recreational and professional, has brought about an increase in the number of sports-related injuries suffered by participants in such activities. Knee injuries, specifically, are common in many sports activities, particularly those requiring running and jumping; affecting weekend sportsmen as well as professional athletes. For example, in sports such as basketball and gymnastics, the knee is subject to the inevitable stress of jumping and landing in awkward or unusual positions which may result in injury and a weakening of the knee. In other sports, knee injuries are similarly common. For example, a football player may fall on an opposing player's fully extended knee or otherwise apply force to the knee structure in a manner and at an angle for which it is not designed, after which the knee may be severely, or even permanently damaged.
During normal movement, the major bone structures proximate to the knee joint rotate within approximately one plane during extension and flexion of the knee. More specifically, these bone structures move in a plane anterior-posterior (front-back) to the body, although there is some medial or lateral (inside or outside) movement, depending upon the knee's activity.
In medical terms, the movement of the knee may be defined as a rotation between the femur (the thigh bone), and the tibia (the shin bone). In flexion, or the flexing of the joint, the tibia rotates toward a smaller angle with the femur. The contrary movement is extension, where the tibia rotates with respect to the femur toward full extension, which is the straightening of the leg. The condyle or articulating surface of the tibia is generally symmetrical, while the condyle of the femur includes an oblong epicondyle with a bone prominence on the posterior side.
At full flexion the tibia rests against the posterior of the femoral epicondyle, creating an overall lengthening of the leg. As the knee begins extension, the tibia travels about a more rounded portion of the femoral condyle, and continues about a progressively flatter portion. Therefore, during rotation toward extension, the respective relationship between the tibia and femur changes because of the change in shape of their adjacent articulating surfaces. As a result, there is an overall shortening of the leg. Specifically, the tibia tends to slide anterior with respect to the femur, and there is an overall shortening of the leg, both of which effects are especially significant during the last 30.degree. of extension. During flexion the reverse is true, i.e., the tibia slides posterior with respect to the femur and the leg becomes longer overall.
To constrain the knee within this range of motion, several ligaments connect the femur with the tibia. Two ligaments are of particular importance; one, the anterior cruciate ligament (ACL), and two, the posterior cruciate ligament (PCL). Generally, the ACL works to prevent the tibia from gliding off the front of the femur during extension and the PCL prevents the tibia from gliding off the rear of the femur during flexion.
One common type of knee injury associated with hyperextension involves damage to the anterior cruciate ligament, which connects the anterior of the tibia with the posterior of the femur. When the ACL is deficient or destroyed, the tibia, during extension can move anteriorly from its anatomically preferred position. An ACL deficiency is a serious problem. In addition to the pain and discomfort caused by injury to the ACL, the misalignment of the joint subjects other ligaments, cartilage, and support structures to increased loads which they were not intended to bear. For this reason, injuries to the ACL are often associated with damage to the medial meniscus cartilage, which can be pinched between the femur and the misaligned tibia. Also due to the misalignment, ACL damage is often accompanied by stretching of the collateral ligaments of the knee, particularly on the lateral side.
In each individual knee injury case, a ligament, or cartilage, or combination of ligaments and cartilage may be damaged. Also, the extent of the damage to any of these knee structures may vary. Accordingly, the recommendation for rehabilitation or healing of the knee may vary depending on the extent and nature of the particular injury suffered by the individual.
Generally, two options are available to those suffering knee injuries of this type: (1) reconstructive surgery, or (2) attempting to rehabilitate the knee through programmed exercise, and the natural healing process. Another option, contraindicated in almost all cases, is the immobilization of the knee for any extended period of time. Such immobilization may in fact be harmful to the healing process, as well as being impractical for day to day life. In addition, a doctor's recommendation that the injured person simply curtail his activities is often not realistic, due to the strong likelihood that the injured person has led an active lifestyle prior to injury and will, in many cases, continue his or her sports activities despite the physician's warning of the risk of further injury.
As stated above, immobilization of the knee for extended periods of time has been shown to have an overall negative effect on the process of healing most injuries commonly sustained by the knee. Conversely, the exercise of an injured knee, in a natural or even programmed manner, often may have very positive effects in the healing of many types of knee injuries. One particular problem overcome by continued exercise is the formation of adhesions, which are defined as "the union of bodily parts by growth." In a nonmoving or immobilized knee, adhesion may occur between the knee joint and cartilage or between the knee bones themselves, among other possibilities. Mobility of the knee joint during the healing process has been proven in clinical studies to reduce the formation of adhesions. Adhesions are particularly troublesome in patients electing reconstructive surgery, and to a lesser extent, the adhesion problem also affects patients not electing surgery.
After sustaining a knee injury of any type, a patient may choose not to submit to surgery for many reasons, including the nature and extent of the injury, his or her medical history, inadequate financial resources, or an unwillingness to undergo a typically protracted convalescence. For those patients not electing surgery, it is extremely desirable to provide the type of knee support that may be provided by an effective orthosis, in order to prevent further injury and to allow the healing process to proceed normally. For certain severely injured patients, a simple, everyday activity such as walking down a flight of stairs may pose the potential for a damaging injury, thereby making the support afforded by an effective knee orthosis a virtual necessity. With a less severely injured knee, an athlete with an effective orthosis may be able to continue physical activity that would not be possible or would be dangerous without the protection afforded by such a knee orthosis.
Even if surgery is chosen, there is need for an effective orthosis. The postoperative knee is typically very weak immediately after surgery. Therefore, for a postoperative knee, an effective orthosis is at least useful, and possibly mandatory in order to protect the repaired structures during the time required for healing. At the same time an effective knee orthosis must allow mobility of the postoperative knee to avoid the above-discussed problem of formation of adhesions, as well as to avoid a number of other problems associated with knee immobility. Furthermore, the natural and normal range of movement is often essential to the proper rehabilitation of the postoperative knee, and thus there is a need for an effective orthosis which allows natural movement while protecting the healing structure.
Therefore, a knee orthosis has been shown to be an extremely useful and sometimes necessary device for a patient suffering an injured knee, whether or not surgery is chosen. An orthosis which can selectively protect the newly repaired ligament or the damaged or weak ligament, while allowing an adjustable range of motion, would greatly advance the rehabilitation process.
Currently, knee orthoses are available which address many of the different types of injuries that may be sustained by the knee, however, none has been able to achieve 100% protection while simultaneously allowing 100% of the normal movement of the knee. In other words, there is always a trade-off between restriction of movement and protection of the damaged knee structure. In addition, some knee orthoses are more effective at correcting specific injuries. For example, an orthosis directed to a knee with a damaged ACL requires force compensation to the anterior of the tibia, while an orthosis for a damaged PCL requires force compensation to the posterior of the tibia. In general, an effective knee orthosis should apply force in such an amount and direction so as to compensate for the function of the damaged knee structures, while simultaneously permitting the largest possible range of movement to the affected knee.
The prior art encompasses many knee orthotic devices. A knee orthosis typically comprises two hinges, one located on the medial (inner) side and the other on the lateral (outer) side of the knee. The orthosis also must include some structure for attaching each of the hinges to the leg, both above the knee and below the knee.
The simplest prior art hinge comprises a single pivot axis, positioned approximately in the middle of the range of motion of the knee. Many such single axis knee orthoses are commercially available.
However, as discussed previously, the actual motion of the human knee does not follow a single pivot point. Rather, during approximately the last 30 degrees of extension, the tibia rolls anteriorly with respect to the femur as much as 8 millimeters, due to the structure of the adjacent contacting surfaces of the femur and the tibia at the knee joint. In addition to the anterior motion of the tibia during extension, the overall length of the leg shortens due to the traveling of the tibia along the posterior condoyle of the femur. Therefore, a leg is at its shortest during full extension, and longest at full flexion.
This shortening of length creates problems for the user of a single axis knee orthosis because such an orthosis cannot lengthen or shorten its overall length. For example, if such an orthosis is securely fastened both to the upper leg and the lower leg at full extension, and then the leg is rotated in flexion, the orthosis cannot become longer like the leg. As a result, in this example, the single axis knee orthosis creates forces pushing the tibia and femur together. This effect, typical of a single axis knee orthosis is commonly termed "pistoning" in the orthotics profession. In many cases, the use of a single axis knee orthosis may actually aggravate the existing injury, or may even increase the risk of other types of knee injury, due in part to the pistoning effect.
Another problem with the single axis knee orthosis is termed "migration" which is also associated with the lengthening and shortening of the leg during flexion an extension. Migration is the partial dislocation of the orthosis due to lengthening and shortening of the leg. Such dislocation, or migration, reduces the effectiveness of the orthosis to protect the injured knee structure, and may contribute to other types of knee injury. In addition, migration may result in lack of comfort to the wearer caused by a loosely fitting knee orthosis that slides about during use.
The polycentric hinge is one type of hinge that has been developed to address the problems of pistoning and migration. A polycentric hinge comprises two fixed pivot points connected by a rigid structural member. Extending outward from each of the two fixed pivot points is another rigid member. Each end remote from the hinge is attached rigidly to the leg respectively above and below the knee. A polycentric hinge is somewhat more effective at avoiding the pistoning problem of the single axis hinge, as it allows for some shortening and lengthening of the knee orthosis in accordance with the shortening and lengthening of the leg. However, such an orthosis is not controllable as to the position in which shortening and lengthening occurs, therefore lacking precise control of the orthosis throughout flexion and extension.
Another disadvantage of the polycentric hinge, common with that of the single axis hinge, is the abrupt halt of the hinge mechanism at the end of extension. In other words, when the knee orthosis reaches full extension, further extension is abruptly halted, transmitting a shock to the knee, which can cause further damage to the already injured knee. In more physical terms, the leg can develop a substantial rotational momentum during extension, which will be partially absorbed by the knee structure which can result in injury to the knee and its surrounding structures.
Other prior art devices designed to simulate motion of the knee include a hinge similar in shape to a cross section of the knee joint, and held together by straps simulating the corresponding ligaments of the knee, described in U.S. Pat. No. 4,361,142 to Lewis et al. Such a hinge consists of a metal, multicurved femoral member in the shape of the sagittal profile of the femur proximate to the knee joint, and a slotted tibial component with a larger, flatter articulating surface approximating the profile of the proximate tibia. Such a hinge approximates the natural motion of the knee, and when inextensible dacron "ligament" straps are connected between the members in various configurations, an approximate model of the knee, complete with restricting ligaments, is established, which is then connected to the body by means of two rigid side bars.
In the device described in the Lewis patent, as in many others, the application of pressure to the various points in the knee, or "loading," is accomplished at or proximate to the hinge and is transmitted to the upper and lower leg, respectively, through a rigid member attached nonrotatably to the upper and lower leg. As a result, such transmittal of force through a rigid member results in a rotation of the orthotic structure affixed to the upper and lower leg, thereby translating to a force applied to the leg via the edge of the orthotic structure. As a result of this edge pressure, there is often an uneven distribution of force throughout the assemblies affixed to the leg both above and below the knee, resulting in discomfort and inefficiencies of the knee orthosis.
In general, it should be remembered that the motion of the knee joint is much more complex than a rotation about a fixed axis and, as shown above, the motion of any knee orthosis is intrinsically less complex than the natural knee motion. Therefore, when a real orthosis is affixed on the leg, there is an inherent conflict between the natural joint structure and the knee orthosis as the orthosis attempts to force the knee to follow its simplified motions. The goal of the knee orthosis designer is to reduce this inherent conflict as much as possible while at the same time providing a restraining force at predetermined limits of motions. These constraints should ideally be introduced in a direction, amount, and location to compensate for the specific knee insufficiency.
However, when such constraints are introduced, their implementation may present a potential for other types of knee injury. For example, as previously discussed, the user of some prior art knee orthoses will encounter an abrupt halt at the end of extension. This abrupt halt, whose severity is increased if the leg has built up substantial momentum in extension, may translate to a substantial shock to the knee and its surrounding structures. In general, the constraint forces of the prior art devices are usually introduced abruptly, at a location proximate to the hinge from which these forces are then translated through a rigid member to the adjacent leg structures. Therefore, it would be an improvement in the art to provide a device which gradually applies an increasingly greater constraint force to avoid the shock that can be encountered when the prior art devices apply constraint forces. It would be a further improvement if the force, or "loading" of the knee orthosis is directly applied proximate to the bone structure where needed to compensate for a particular injury.
For example, in injuries to the ACL, the structure of the knee, together with the momentum building up in the extending leg, create an increasingly substantial force in extension which can move the tibia anteriorly off the femur, misaligning the joint and thereby causing a potential for injury to other knee structures. Therefore, it can be seen that it would be an improvement in the art to provide an orthosis to compensate for an ACL injury by applying an increasing force during extension to properly align the tibia with respect to the femur. It would be a further improvement if such a knee orthosis were to apply constraint forces directly to the structures surrounding the knee, so that the increasing constraint force counteracts the anterior forces on the tibia, thereby providing substantial protection to the ACL deficient knee. It would be a still further improvement if such a knee orthosis could avoid the previously discussed problems of pistoning and migration, by shortening during full extension and lengthening during flexion.
Finally, the effective prior art devices operate in a single plane throughout extension. For example, the single axis hinge and the polycentric hinge are both constrained to move in a single plane by the nature of their connections. Therefore, these prior art orthoses allow only knee movement within this one plane, while the natural motion of the knee, although moving generally in one plane, actually moves in three dimensions and rotates about its axis, if permitted. It is of interest that, in general, the rotary ligaments of the knee are the controlling factors of three dimensional motion. The excursions of the joint from the single plane are generally small and depend upon the activity the knee is supporting. Although such excursions are small they are substantially not permitted by prior art devices that have a fixed axial structure, such as a single axis knee orthosis. Therefore, if the rotary ligaments are not damaged, then in many cases the prior art orthoses restrict movement unduly.
Therefore, it would be an improvement in the art to provide such a device that allows a slight rotary movement of the tibia with respect to the femur, so as to permit a more natural movement of the knee joint.